1. Field of the Invention
The present invention relates to a process for a phosphate chemical conversion treatment of a steel material which may be a surface-treated steel sheet, for example, a zinc-plated steel sheet. More particularly, the present invention relate to a process for the phosphate chemical conversion treatment of a steel material by using a specific phosphate chemical conversion treating liquid at room temperature to form a phosphate coating layer firmly fixed on the steel material.
2. Description of the Related Arts
With aspect to treatment temperature, phosphate chemical conversion treatment baths are classified into a room (ambient atmospheric) temperature treatment bath and a high temperature treatment bath. The high temperature treatment bath is usually used while heating the bath at a temperature of more than 40.degree. C., and the conventional phosphate chemical conversion treatment bath used for pretreating parts of cars prior to painting is usually a high temperature treatment bath. The room temperature treatment bath is usually used at a temperature of 40.degree. C. or less, preferably 35.degree. C. or less but 0.degree. C. or more, without external heating.
U.S. Pat. No. 4,233,087 discloses a process for the room temperature chemical conversion treatment. In this process, wherein an acid phosphate chemical conversion treatment bath containing hydrogen peroxide as an oxidizing agent is used, a molar ratio (PO.sub.4 /Zn) of phosphate ions to metal (zinc) ions is maintained in a range of from 0.5 to 3.7, and the phosphate chemical conversion treatment is smoothly carried out so that even if an additional feed is introduced into the bath, the chemical conversion treatment can be stably effected at room temperature. To enable the molar ratio (PO.sub.4 /Zn) to be maintained at the predetermined level, a certain amount of N.sup.n- ions, which are a diluting agent for the anions and are selected from NO.sub.3.sup.-, SO.sub.4.sup.2- and Cl.sup.- ions, must be kept in the treating bath. In the process of the above-mentioned U.S. patent, preferably, the pH of the bath is at a level of about 3.0 and the ratio in weight of the phosphate ions to the entire mixed anions in the conversion treating bath is 70% or more.
U.S. Pat. No. 4,565,585, an inventor of which is one of the inventors of the present invention, relates to a phosphate chemical conversion treatment process at room temperature. This process is characterized in that the phosphate chemical conversion treatment is carried out at a specific level of pH and oxidation-reduction (redox) potential (ORP) so that a general electrochemical corrosion reaction can occur on an entire surface of the steel material, and thus a phosphate chemical conversion coating layer can be formed on the steel material surface. In an example of U.S. Pat. No. 4,565,585, the ratio of the weight of phosphate ions to the entire weight of the mixed anions in the chemical conversion treating liquid is in the range of from 70% to 80%.
U.S. Pat. No. 4,657,600 discloses a process of phosphate chemical conversion treatment for a steel material with a treating liquid containing metal ions, oxacid ions, and phosphate ions, and having a pH and an oxidation-reduction potential (ORP) adjusted to a predetermined level, respectively, without directly replenishing nitrite ions as an oxidizing agent. The principal chemicals comprise the above-mentioned ions. In this process, the oxidizing agent such as nitrite ions must not be directly added to the principal chemicals. When this type of oxidizing agent is mixed with principal chemicals of the chemical conversion treatment before they are fed into the chemical conversion treating liquid, the principal chemicals react with the oxidizing agent to a large extent, and therefore, the oxidizing agent must be added to the chemical conversion treatment liquid separately from the principal chemicals, namely, the phosphate ions, metal ions, oxacid ions, etc.
As disclosed in U.S. Pat. No. 4,565,585, the phosphate chemical conversion coating layer-forming reaction at room temperature comprises an electrochemical anodic reaction which causes iron to be dissolved, and a chemical conversion coating layer-forming reaction for producing iron phosphate and zinc phosphate. That is, in the first step of the phosphate chemical conversion coating layer-forming reaction, a portion of iron located in the surface portion of the steel material is dissolved in accordance with the conversion: Fe.fwdarw.Fe.sup.2+ +2e, and after the portion of iron is dissolved to form iron ions, reactions of phosphate ions with iron ions and zinc ions occur on the surface of the steel material.
When considered from a thermodynamic point of view, the above-mentioned chemical reactions progress in a direction in which the Gibbs free energy (.DELTA.G) of the whole reaction system is decreased. This .DELTA.G is defined by the following equation (1): EQU .DELTA.G=.DELTA.H-T.DELTA.S (1)
wherein .DELTA.H represents an enthalpy of the reaction system, T represents an absolute temperature of the reaction system, and .DELTA.S represents an entropy of the reaction system.
The equation indicates that a decrease of the .DELTA.G of the reaction system is realized by decreasing the .DELTA.H of the reactions or by increasing the .DELTA.S. When an external energy is imparted, i.e., heating, the .DELTA.H of the reaction system increases, and thus the reaction progresses in a direction of increasing the .DELTA.S. That is, in a high temperature treating liquid, the .DELTA.S increasing reaction is carried out in accordance with reaction (2): EQU H.sup.+ +e.fwdarw.1/2H.sub.2 ( 2)
As a result of the reaction (2), the concentration of H.sup.+ ions in the high temperature treating liquid decreases, and thus dissociation of the phosphoric acid is promoted. However, in the room temperature treating liquid, reaction (2) is difficult to obtain.
In U.S. Pat. No. 4,565,585 the inventor of the present invention made it clear that an important difference between the room temperature and high temperature phosphate chemical conversion treatments resides in the reactivity of reaction (2), and disclosed a specific method for practically utilizing the specific reactions in the room temperature phosphate chemical conversion treatment liquid.
The inventors of the present invention studied the conventional room temperature phosphate chemical conversion treatment process from the view point of an etching of the steel material to be treated.
In comparison with a high temperature treatment liquid which is heated, the phosphate ions in the room temperature treatment liquid, which is not heated, are in a non-activated state. Phosphoric acid has a relatively low degree of dissociation, and thus is included in a weak acid group having a low activity.
Generally, phosphate ions, other types of anions, and cations have a higher activity in a high temperature treatment liquid than that in a room temperature treatment liquid. Also, generally, when a content of phosphate ions based on the weight of the total of mixed anions is high, the resultant phosphate chemical conversion treatment liquid exhibits high stability compared to a room temperature treatment liquid having a low content of phosphate ions. However, the stable treatment liquid exhibits a low chemical activity, and thus is not adequate for etching a steel material.
Here it is important to note that the phosphate chemical conversion reaction in any of the room temperature and high temperature treatment liquids of any type of phosphate chemical conversion treatment compositions can be understood as being a phosphate chemical conversion coating layer-forming reaction derived from a dissolution of iron from the steel material. The research of Maclu, as disclosed in The Journal of the Metal Finishing Society of Japan, Vol. 20, No. 5, pages 39 to 42, 1969 and evaluated as a most authoritative study on the phosphate coating layer-forming reactions, analyzed in detail the reaction in the conventional high temperature treatment liquid. In this analysis, the phosphate coating layer-forming reaction was illustrated as a reaction derived from the dissolution of iron in the steel material. In the conventional high temperature treatment liquid, generally, the weight ratio of the phosphate ions to the total of the mixed anions is relatively high, but since the total amounts of ions in the treatment liquid at an elevated temperature are active, the dissolution of iron from the steel material is promoted.
In a conventional room temperature phosphate chemical conversion treatment liquid disclosed in U.S. Pat. No. 4,565,585, which contains phosphate ions in a weight ratio of more than 50% to the total of mixed anions, where the steel material is very weakly etched, there is a disadvantage in that, even if a phosphate chemical conversion coating layer is formed on the steel material, the adhesion of the coating layer to the steel material is not strong. To enhance the etching effect on the steel material, if the concentration of phosphate ions is excessive compared with that in the conventional treatment liquid, the resultant treatment liquid exhibits an excessively low pH and has and unbalanced composition, and thus cannot form an effective phosphate chemical conversion coating layer on the steel material. The above-mentioned disadvantages appear particularly when the treatment is carried out by immersion. Where the treatment is carried out by spraying, the steel material always comes into contact with a fresh treatment liquid, the reaction of the steel material with the treatment liquid is in a gas phase, and thus the etching of the steel material is properly effected.
As described in U.S. Pat. No. 4,565,585, when a room temperature treatment liquid containing an excessively large amount of phosphate ions is applied by spraying, the etching effect of the room temperature treatment liquid is smaller than that of the high temperature treatment liquid. However, in the spraying, since the steel material can always come into contact with a fresh active treatment liquid in a gas phase, the treatment liquid can properly react with the steel material and easily form a phosphate chemical conversion coating layer firmly fixed on the steel material.
In the immersion method, the steel material cannot always come into contact with a fresh treatment liquid, and all the reactions are carried out in a liquid phase. Accordingly, when a conventional room temperature treatment is carried out by immersion, an extra method is needed to obtain a firmly fixed phosphate chemical conversion coating layer. Usually, when the conventional treatment is carried out by immersion, using a phosphate chemical conversion treatment liquid containing an excessively large amount of phosphate ions, it is difficult to etch the steel material to a satisfactory extent and to form a phosphate chemical conversion coating layer firmly fixed on the steel material.
In a practical room temperature phosphate chemical conversion treatment, it is important to satisfy the following requirements:
(A) The treatment can be carried out in a continuous manner. PA1 (B) The conditions of the treatment liquid for the phosphate chemical conversion can be continuously controlled. PA1 (C) To realize the above requirements (A) and (B), an undesirable generation of sludge in the treatment liquid is prevented or restricted.
However, a room temperature phosphate chemical conversion treatment process by immersion which fully satisfies the above mentioned requirements has not been found.